Electro-magnetic devices

ABSTRACT

An electro-magnet includes a pole-piece having a portion of reduced cross-section. The shape of the reduced portion provides a restriction in the magnetic circuit such that the distance moved, against a biasing force, by an armature of the electro-magnet is proportional to the current in the electro-magnet winding. The pole piece comprises aligned parts of nickel-iron alloy joined by a soft iron part whose cross-section is less than that of the nickel-iron alloy parts.

This invention relates to electro-magnetic devices.

It is known, from British Patent Specification No. 1,024,112, to providean electro-magnetic device having a central pole piece surrounded by awinding, the pole piece being formed intermediate its ends with awaisted portion to provide a restriction in the magnetic circuit suchthat the distance through which an associated armature is moved, againsta biasing force, is substantially proportional to the current flowing inthe winding. It is the normal practice to make the central pole piecefrom a nickel iron alloy, such as that commercially available under thename "Permalloy B. " The flux density at which nickel iron alloysaturates decreases with increasing temperature, there being, typically,a 9% reduction in the saturated flux density between 20° C and 180° C.Thus, to maintain a given flux density in an air gap between the polepiece and the armature, a substantial increase in the current in thewinding is required at elevated temperatures. It is an object of thepresent invention to provide an electro-magnetic device in which theforegoing disadvantage is substantially reduced.

According to the invention an electro-magnetic device includes a centralpole piece, a winding surrounding said pole piece, an armatureattractable into contact with the pole piece and a spring urging thearmature away from the pole piece, the pole piece having first andsecond axially aligned parts of nickel iron alloy and a soft iron partjoining said first and second parts and having a cross sectional arealess than that of either said first or said second part, said soft ironpart defining, intermediate the ends of said pole piece, a waistedportion which provides a restriction in the magnetic circuit of thedevice, the shape of said waisted portion being such that the distancethrough which the armature moves against the action of the spring issubstantially proportional to the current flowing in the winding.

In a preferred embodiment said first and second parts and said soft ironpart are substantially cylindrical.

In a further preferred embodiment said soft iron part is engaged inrecesses in adjacent end faces of said first and second partsrespectively.

An example of the invention will now be described with reference to theaccompanying drawing.

The magnetic device shown has a central pole piece 10 having an integralflange 11. The pole piece 10 supports an insulating former 12 aboutwhich there is a winding 13. An annular member 14 of magnetic materialengages the flange 11 and surrounds the winding 13. An armature 15 ismounted for pivotal movement with respect to a lever 16. Lever 16 canmove about a spindle 17 and is biased by a spring 18 against an abutment19 so as to urge armature 15 away from the pole piece.

Pole piece 10 is formed in three parts. First and second parts 20, 21are in the form of cylindrical tubes and are axially aligned in spacedrelationship. Parts 20, 21 are formed of a nickel-iron alloy of the typecommercially available under the name "Permalloy B," this alloy havingthe characteristics of high permeability and low hysteresis. Between theparts 20, 21 is a soft iron bush 22 which engages recesses in theadjacent faces of the parts 20, 21 so as to maintain the latter in theiraxially aligned, spaced relationship. The end face of part 21 which isadjacent the part 22 is relieved, as shown, to provide a projectingfrusto-conical surface co-axial with the axis of the pole piece 10 andhaving an included angle of between 130° and 160°.

The bush 22 and adjacent faces of the parts 20, 21 define a waistedportion of the pole piece 10. The shape of the waisted portion is suchthat after magnetic saturation has taken place in the neck defined bythe soft iron bush 22, any further increases in magnetising force causesthe circuit flux to leak across a progressively wider space between theparts 20, 21. Thus, after initial saturation of the bush 22, increasingmagnetising force produces an effective increase in the pole piece crosssection. The effective reluctance of the magnetic circuit, however,increases with the effective cross section, due to the wider gap betweenparts 20, 21 which the flux must cross.

The dimensions of the bush 22 and parts 20, 21 are so chosen that theeffect is to correct the tendency, in a device of this kind, for theforce exerted against the spring 18 to rise steeply as the air gapbetween the pole piece 10 and armature 15 decreases, and in the deviceof the present invention an almost linear force/distance curve results.In the present example outside diameters of the portions 20, 21 are17.55 mm and the outside diameter of bush 22 is 9.14 mm.

It has been found that with this type of pole piece an electro-magneticdevice of the kind described has a reduced sensitivity to increases intemperature. The axial length of bush 22 is maintained at the minimumnecessary to effect the connection between parts 20, 21. The advantagesof low hysteresis and high permeability obtained with "Permalloy B" areretained.

I claim:
 1. An electro-magnetic device, including a central pole piece, a winding surrounding said pole piece, an armature attractable into contact with the pole piece and a spring urging the armature away from the pole piece, the pole piece having first and second axially aligned parts of nickel iron alloy and a soft iron part joining said first and second parts and having a cross sectional area less than that of either said first or said second parts, said soft iron part defining, in combination with adjacent ends of said first and second parts, a waisted portion intermediate the ends of said pole piece, said waisted portion providing a restriction in the magnetic circuit of the device, the shape of said waisted portion being such that the distance through which the armature moves against the action of the spring is substantially proportional to the current flowing in the winding.
 2. A device as claimed in claim 1 in which said first and second parts and said soft iron part are substantially cylindrical.
 3. A device as claimed in claim 1 in which said first and second parts and said soft iron part are tubular.
 4. A device as claimed in claim 1 in which said soft iron part is engaged in recesses in adjacent end faces of said first and second parts respectively.
 5. A device as claimed in claim 4 in which one of said end faces is a frusto-conical surface which projects towards the other of said end faces.
 6. A device as claimed in claim 5 in which said frusto-conical surface has an included angle of between 130° and 160°. 